Allow overriding font for letters in math mode

Traditionally, most TeX systems make a distinction between math mode and text mode, and different fonts may be used for each.

In XAML-Math, we use Computer Modern for math mode, and any user-provided font for text mode (i.e. the `\text{}` command).

The reason we use Computer Modern is that it includes the special characters to properly draw the radical character, several symbols for brackets of different sizes and so on.

Notably, we _do not_ rely that much on actual _letters_ (or, for that matter, digits) from the font.

Also, we now have a way of generating certain font size metrics on the fly.

I suggest us to consider providing an option to override the font used for text in the math mode.